Post thumbnail image doesn't show without featured image

Hi,
I ve been working on this issue for a while now.
I moved my blog to a self-hosted WP and since the beginning,
the option to use last image of the post as thumbnail if there is no featured image is not working.
– If i set a featured image, everything is fine
– If there is no featured image, Customizr doesnt use the last or any picture in the post
– I’ve activated the option, turned it off, turned it on, multiple times – no change
– I deactivated all plugins – no improvement

Any ideas anyone?

Do I have to adjust anything additionally in any .php file?

If someone might help by looking at the page, yu find it here:
The Horizon is Our Home https://www.thehorizonisourhome.de/

There are several posts on the front page, the post that are displaying thumbnails are the ones where I set a feautred image already.

Thx in advance!
Henning